One example of a motor that drives industrial machinery is a synchronous magnet motor. A synchronous magnet motor generally requires a current control operation in accordance with the magnetic pole position of the rotor. An attempt to rotate the motor during unstable magnetic pole position of the motor reduces the output torque per unit current of the motor, and thus a desired output characteristic may not be achieved. The magnetic pole position of a motor can be calculated from positional data of a detector if the attachment position relationships between the detector and the motor magnetic poles are known. However, the attachment position relationships between the detector and the motor magnetic poles may vary from motor to motor. In this case, the magnetic pole position cannot be detected accurately from positional data, thereby requiring estimation of the magnetic pole position.
In addition, the motor that drives industrial machinery may be used for a horizontal shaft or a gravity shaft. In a case of use for a horizontal shaft, a method is proposed in which a current sufficient to actually drive the motor is supplied, and the rotational operation of the motor is then observed to estimate the magnetic pole position. Otherwise, in a case of use for a gravity shaft, to prevent dropping of the gravity shaft, the magnetic pole position of the motor needs to be estimated while the motor is mechanically restrained, that is, while the motor is in a stopped state. This presents a problem in that, when a motor is used for a gravity shaft, the magnetic pole position cannot be estimated from the motor operation.
To address the above problem, various motor control devices have been proposed that estimate the magnetic pole position while the motor is in a stopped state even if the motor is mechanically restrained.
One example of a motor control device that estimates the magnetic pole position while the motor is in a stopped state is described in Patent Literature 1 below. Patent Literature 1 discloses a technology in which measurement is made on the motor current when a voltage pulse output from the inverter is applied to the motor so as to estimate the magnetic pole position on the basis of the difference in the sum of the current amplitudes caused by the magnetic saturation characteristic of the winding. The technique of Patent Literature 1 causes a current to flow through the motor winding, but the current value is low, thereby avoiding generating a torque that rotates the motor. Thus, even if the target motor is connected to a fixed shaft mechanically restrained, the magnetic pole position can be estimated.